This invention relates to a method of monitoring and controlling the charging of gastight alkaline rechargeable batteries and determining critical states of charge, particularly to a method that can monitor charging based on voltage and temperature.
Rechargeable alkaline battery systems are used in large quantities for modern equipment applications. In addition to these applications, they will also be increasingly used in the future in vehicles, both as a propulsion battery in hybrid vehicles and as batteries for vehicle power supply systems. High power output and the capability of feeding electrical energy back at high power effectively are essential characteristics of alkaline systems.
Of the alkaline secondary systems: nickel cadmium (NiCd), nickel metal hydride (NiMH), nickel zinc (NiZn), and nickel iron (NiFe), the nickel metal hydride system has been found to be the system having the best characteristics. In comparison with other alkaline secondary systems, it has a better charge capacity, longer life and avoids the feared xe2x80x9cmemory effectxe2x80x9d. In addition, it does not make use of toxic heavy metals.
The capabilities for rapid charging of alkaline secondary systems extend down to the range of a few minutes. Rapid charging is limited by critical cell voltages being exceeded, which are governed by the decomposition voltage of the water and oxygen gassing at the positive electrode, whose capacitance with respect to the negative electrode is underdimensioned in a gastight alkaline cell. Oxygen gassing at the positive electrode takes place as a parasitic reaction when the positive electrode is approaching the fully charged state, and is the reason why it is necessary to limit the charging current.
The oxygen gassing reaction can result in pressure building up in the cell which, in the worst case, leads to safety valves operating and to charging gases and electrolyte escaping. Since both can have a negative effect on the life expectancy of the gastight cells, it is desirable to identify such critical states of charge at an early stage, and to limit or cut off the charging currents in good time.
However, identification of the critical states of charge is problematic. A pressure measurement is regarded as being too complex. Only the cell voltage and temperature are available as variables which can be measured from outside the cell. Since oxygen gassing reactions in gastight alkaline secondary systems are accompanied by an exothermal oxygen dissipation reaction at the negative, opposing electrode, the rate of temperature rise, which is normally observed, is in general also a signal of the start of gassing and, thus, that the pressure inside the cell is rising. However, particularly with very high charging currents, the temperature signal can be used only to a limited extent since the high thermal capacity of aqueous battery systems leads to the temperature rising only relatively slowly as a consequence of the onset of overcharging.
Thus, it would be highly advantageous to provide a method for monitoring the charging and determining critical states of charge, in which only the voltage and temperature of the rechargeable battery to be monitored are measured and are used for assessment.
This invention relates to a method of controlling charging of a gastight alkaline rechargeable battery including determining characteristics of critical charging voltage (Ucrit) of the rechargeable battery as a function of charging current (I) at selected temperatures (T), linearizing the critical charging voltage according to the follow formula: Ucrit=A(I)xc3x97T+B(I), wherein A=xcex94Ucrit/T/V and B=Ucrit at 0xc2x0 C./V, and wherein A and B are stored as parameter arrays in a battery management system containing a substantially physically identical rechargeable battery, calculating an associated critical charging voltage in the substantially physically identical rechargeable battery by measuring temperature and charging current, and comparing associated critical charging voltage information with the critical charging voltage of the rechargeable battery to control the charging of the rechargeable battery.